The recent development of lasers such as excimer lasers has increased the demand for optical systems and components that can transmit ultraviolet light. Such optical systems and components typically require antireflection coatings effective against grazing incident light, including "grazing" incident light (i.e., incident light having a high angle of incidence .theta. of generally 70.degree. or greater).
An excimer laser produces linearly polarized light. Depending upon the placement of the optical system relative to the excimer laser, light from the laser enters the optical system as either p-polarized light or s-polarized light.
Conventional multilayer antireflection coatings for grazing incident light consist of stacked layers in which layers of high-refractive-index material and low-refractive-index material are alternatingly arranged. A larger number of layers is required as the angle of incidence is increased (e.g., whenever .theta..ltoreq.70.degree., the number of layers is four or more).
Conventional multilayer antireflection coatings also typically have a non-periodic multilayer structure in which the optical thickness of each layer is different. Hence, the optical thickness of each layer must be precisely controlled when forming such an antireflection coating. Also, for each incidence angle, the optical thickness of each layer and the number of layers in the antireflection coating must be different.
In making a conventional multilayer antireflection coating for grazing incident light (the coating being made with a non-periodic structure), the following difficulties are encountered: (1) During formation of each of the stacked layers, variations in layer thickness frequently arise; such variations cause corresponding unplanned variations in refractive index over the antireflective coating. Hence, it is necessary to evaluate the refractive index of each layer before deposition. (2) If the optical performance of a coating is different from ideal performance, it is difficult to identify the specific culprit layer(s) in the coating because the optical thickness of each layer is different. Consequently, obtaining a coating that is entirely within specification requires a trial-and-error approach. (3) Parameters that must be controlled to controllably form the correct optical thickness of each layer of the multilayer antireflection coating are complicated. (4) For each angle of incidence, both the optical thickness of each layer and the total number of layers in the multilayer antireflection coating must be different.